Metallic coating for wire



United rates Patent METALLIC coA'rrNt; non Winn Charles L. Faust, Columbus, @hio, assignor to the United States of America as represented by the Secretary of the Army No Drawing. Application Aprii 3, 1952, Serial No. 280,361

2 Claims. (Cl. 204-12) This invention relates to coatings for electric conductors and more particularly to substitute coatings for tin on copper wire.

Because tin is a critical material in times of war, it is essential that its use be minimized as much as possible. A substantial saving can be realized by replacing the tin coatings on copper wire with a non-critical material.

The substitute for the tin coating should possess approximately the same soldering properties as tin, and it should serve as an adequate barrier between the copper Wire and the insulation. In addition, the coating should function satisfactorily in a wide temperature range, preferably in the range of about 50 C. to 200 C. To enable the use of the coating in all types of climatic conditions, particularly in tropical exposures, high humidity resistance and resistance to fungal growth are important,

A most necessary function of the barrier between the copper and the insulation is the prevention of attack by constituents in the insulating material as, for example the prevention of the so-called copper effect on rubber insulation. Here, the free sulphur in the rubber insulation attacks the copper, and the compounds formed and the copper itself react with the rubber to deteriorate it. Where the insulation is of an organic-chloride type such as vinylite, there is the possibility of attack on the copper by chlorine.

It is therefore a primary object of this invention to provide a tin-free alloy which will serve as a substitute for a tin coating on copper wire, such alloy possessing approximately the same soldering properties as tin and serving as an adequate barrier between the copper wire and the insulation.

It is another object to provide a process for co-electrodepositing an alloy having the aforementioned qualities.

It is a further object to provide an electrolytic bath capable of producing such an alloy.

These and other objects and advantages of the present invention will be better understood as the detailed description thereof progresses.

In accordance with the present invention, there is provided a Wire conductor having an electrodeposited metallic coating thereon, said coating comprising an alloy containing lead and antimony. A preferred embodiment of said coating is provided comprising about 88% lead and about 12% antimony.

Also, in accordance with the present invention there is provided a method of electrodepositing a lead-antimony alloy directly upon a wire conductor comprising immersing the wire conductor to be plated in an aqueous solution consisting of basic lead acetate, 21 hydroxide selected from the group consisting of potassium, sodium and ammonium hydroxides, a tartrate selected from the group consisting of potassium, sodium and ammonium tartrates, sodium potassium tartrate and an antimonyl tartrate selected from the group consisting of potassium, sodium and ammonium antimonyl tartrates and passing an electric current through the solution in such a manner so as to make the wire conductor to be plated a cathode.

2,718,494 Patented Sept. 20, 1955 It has been found as a result of the present invention that a coating of a lead antimony alloy, electrodeposited directly upon copper possesses excellent solderability which is equivalent to the solderability of tinned copper Wire, adheres well to the underlying copper, and has a satisfactory continuity of coating. A percentage distribution of approximately eutectic composition in the leadantimony alloy (88 Pbl2 Sb) is preferred.

To produce copper wire coated with Pb-Sb alloy by means of electrodeposition, the copper wire to be coated is made the cathode, a steel plate is made the anode and the electrolyte used is a dilute alkaline lead bath to which potassium antimonyl tartrate (tarter emetic) and potassium tartrate has been added.

The method used to produce this alloy and apply it directly to copper wire is as follows:

Sixteen-gauge copper wire (0.05082 inch diameter) is straightened by stretching a length of wire from 33 inches to 36 inches in length by means of a vise and a pair of pliers. This wire cathode is thoroughly cleaned and cut into five lengths of straightened wire, each length being six inches in active cathode length. The wires are so racked that they lie in a vertical plane with their axes 0.75 inch apart.

The anodes consist of steel plates 0.06 inch thick X 3 /2 inches wide x 6 inches immersion length. Two anodes are used, the anodes being spaced two inches on each side of the cathode.

The plating bath consists of 2.8 liters of solution and consists of the following ingredients; 7.5 grams per liter of basic lead acetate, 18 grams per liter of sodium hydroxide, 3.5 grams per liter of sodium potassium tartrate, 19 grams per liter of potassium antimonyl tartrate, 12 grams per liter of potassium tartrate, and 0.7 cc. per liter of a betaine (RH 556 Du Pont Product). The plating bath is maintained at F. to F. during the electrodeposition and the current density applied during the plating process is 10 amperes per square foot. The cathode is agitated in the direction of the plane of the wires (by work-rod movement) at the rate of 66 reciprocal strokes per minute, each stroke being 4 inch in length.

Under the above stated conditions, the time to produce a coating thickness of 0.00001 inch is 17 seconds, 0.00002 inch is 34 seconds, 0.00004 inch is 68 seconds, etc.

The'coatings on the copper wire of an eutectic composition of Pb-Sb alloy (88 Pb-l2 Sb) has been found to .be a very effective substitute for tin having excellent characteristics being sought in a substitute coating for tin'was tested as follows:

To determine solderability the following procedure was used. A pair of 16 guage wire specimens were clamped in a vise at the ends and twisted together (7 complete turns for 7 inch long specimens). The twisted wires were cut to a 6 inch length and placed in a symmetrical jig clamped at the middle of the wires. One end of the twisted wire was immersed in a solution of 40 grams of wood rosin in cc. of ethyl alcohol. By means of the jig, the fluxed ends of the wires were suspended with 0.4 inch immersion in a 50 tin-50 lead solder bath for 30 seconds at 312 C. (503 F.). The wood rosin was removed with xylene and the capillary rise of the solder, above the point of contact with the solder bath, was measured to the nearest 64th of an inch.

In the above test, the antimony-lead eutectic (12 Sb- 88 Pb) coating with a thickness of 0.00004 inch, showed a capillary rise of inches. A standard tin coating, 0.00006 inch thick, tested by the same method showed a capillary rise of only inch.

To determine the continuity of coating, the test was made in accordance with ASTM Standard Specifications for tinned copper wire. The solutions required for the making of this test and their preparation is described hereunder.

A test solution of hydrochloric acid with a specific gravity of 1.088 was required. This was provided as follows. A commercial hydrochloric acid (specific gravity 1.12) was diluted with distilled water to a specific gravity of 1.088 measured at 155 C. (60 F.). A ISO-milliliter test portion of the dilute acid was considered to be exhausted when 10 test specimens of 16-gage wire had been immersed in it for two cycles.

A sodium polysulphide solution with a specific gravity of 1.142 was also required for this test and was prepared in the following manner. A concentrated solution was prepared by dissolving sodium sulphide crystals in distilled water until the solution was saturated at about 21 C. (70 F.), adding 250 grams per liter of flowers of sulphur, and allowing the saturated solution to stand for at least 24 hours. The test solution was made by diluting a portion of the concentrated solution with distilled water to a specific gravity of 1.142 at 155 C. (60 F.). The sodium polysulphide test solution had to have suflicient strength to blacken thoroughly a piece of clean, untinned copper wire in seconds. A portion of the test solution used for testing samples was considered to be exhausted when it failed to blacken a piece of clean copper.

To test the continuity of the coating on the copper wire, a length of at least 4 /2 inches from each of the clean specimens was immersed in accordance with the following cycles in test solutions maintained at temperatures between 15.5 C. and 21 C. (60 and 70 F.). The specimen was immersed in the hydrochloric acid solution described above, washed and wiped dry. The specimen was then immersed for 30 seconds in the sodium polysulphide solution described above, washed and wiped dry. The specimen was again immersed for 1 minute in the hydrochloric acid solution, washed and wiped dry. Finally the specimen was immersed for 30 seconds in the sodium polysulphide solution, washed and wiped dry.

After each immersion, the specimens had to be immediately and thoroughly washed in clean water and wiped dry with a clean, soft cloth.

After the operations described above, the specimens were examined to ascertain if any copper was exposed through openings in the coatings as revealed by blackening action of the sodium polysulphide. Such blackening of exposed copper indicated failure of the coating.

The wire specimens coated with the electrodeposited Pb-Sb alloy when subjected to the continuity of coating test did not reveal any blackening due to exposed copper in the coating.

To test adhesion of the coating material, the coated wire to be tested was bent around a rod having a diameter equal to four times the diameter of the wire, and the bent portion was dipped in the hereinabove described sodium polysulphide solution for 30 seconds. Any cracking or parting of the coating, shown by blackening of the copper, was indicative of failure of the coating. In this test, the Pb-Sb coating showed no cracking and blackening.

The aging test consisted of providing a uniform temperature of 200 C. and an atmosphere of inert gas around the wires for 30 hours. At the end of this period, the appearance and adhesion of the coating was observed and the solderability of the coatings was determined. Helium from commercial cylinders was found to give satisfactory results as the inert gas for the atmosphere about the wires. The Pb-Sb coating showed results in this test equal to the test results of a tin coating.

In summary, this invention provides an alloy made up of lead and antimony and a process for coating copper wire directly therewith. The alloy is an effective substitute for tin coating on copper wire. A eutectic leadantimony alloy which is a composition consisting of about 88% lead and 12% antimony is the best combination. The latter compares very favorably with tin with respect to solderability, adhesion to the underlying copper wire, continuity of coating, and durability with respect to aging. Inasmuch as all hookup wire and, in many cases, field wire has to be tinned, it is quite obvious that this invention can effect a great saving in tin.

While there have been described what at present are considered to be preferred embodiments of the invention, it will be understood by those skilled in the art that various changes and modifications may be made herein without departing from the invention and it is therefore aimed in the appended claims to cover all such modifications as fall within the spirit and scope of the invention.

What is claimed is:

1. In the method of co-electrodepositing a lead and antimony alloy coating directly upon copper wire said coating containing about 12% by Weight of antimony and 88% by weight of lead, comprising immersing the copper wire in an electrolytic bath consisting substantially of an aqueous solution of about 7.5 grams per liter of basic lead acetate, 18 grams per liter of sodium hydroxide, 3.5 grams per liter of sodium hydroxide tartrate, 19 grams per liter of potassium antimonyl tartrate, and 12 grams per liter of potassium tartrate, passing an electric current through the bath in such a manner so that the copper wire to be plated is made the cathode, regulating the electric current to provide a current density of about 10 amperes per square foot and heating the bath to a temperature of about F. to F.

2. An electrolytic bath for coelectrodepositing a leadantimony alloy coating comprising about 88% by weight of lead and 12% by weight of antimony directly on copper wire, comprising substantially 7.5 grams per liter of basic lead acetate, 18 grams per liter of sodium hydroxide, 3.5 grams per liter of sodium potassium tartrate, 19 grams per liter of potassium antimonyl tartrate and 12 grams per liter of potassium tartrate.

References Cited in the file of this patent UNITED STATES PATENTS 713,568 Lloyd Nov. 11, 1902 1,405,535 Merritt Feb. 7, 1922 1,509,101 Dana Sept. 23, 1924 2,184,179 Domm Dec. 19, 1939 2,472,296 Hartnell June 7, 1949 2,545,566 Booe Mar. 10, 1951 2,555,375 Ruemmler June 5, 1951 2,634,235 Hitchens Apr. 7, 1953 FOREIGN PATENTS 130,302 Great Britain July 29, 1919 229,932 Great Britain Mar. 5, 1925 OTHER REFERENCES The Metal Industry, vol. 18, N0. 6, June 1920, pages 264-266.

Monthly Review Amer. Electro Platers Soc., Oct. 1942, page 870. 

1. IN THE METHOD OF CO-ELECTRODEPOSITING A LEAD AND ANTIMONY ALLOY COATING DIRECTLY UPON COPPER WIRE SAID COATING CONTAINING ABOUT 12% BY WEIGHT OF ANTIMONY AND 88% BY WEIGHT OF LEAD, COMPRISING IMMERSING THE COPPER WIRE IN AN ELECTROLYTIC BATH CONSISTING SUBSTANTIALLY OF AN AQUEOUS SOLUTION OF ABOUT 7.5 GRAMS PER LITER OF BASIS LEAD ACETATE, 18 GRAMS PER LITER OF SODIUM HYDROXIDE 3.5 GRAMS PER LITER OF SODIUM HYDROXIDE TARTRATE, AND 12 GRAMS PER LITER OF POTASSIUM ANTIMONYL TARTRATE, AND 12 GRAMS PER LITER OF POTASSIUM TARTRATE, PASSING AN ELECTRIC CURRENT THROUGH THE BATH IN SUCH A MANNER SO THAT THE COPPER WIRE TO BE PLATED IS MADE THE CATHODE, REGULATING THE ELECTRIC CURRENT TO PROVIDE A CURRENT DENSITY OF ABOUT 10 AMPERES PER SQUARE FOOT AND HEATING THE BATH TO A TEMPERATURE OF ABOUT 75* F. TO 85* F. 